The overall objective of this project is to study molecular mechanisms responsible for the pathogenesis of human parainfluenza virus type 3 (HPF3). Previous work in the laboratory of the proposed mentor has identified the role of the HPF3 envelope receptor-binding glycoprotein hemagglutinin-neuraminidase (HN) in the virus-induced fusion process. The HN-receptor interaction is critical for several essential components of the viral life cycle - entry, fusion and release - and this interaction regulates pathogenicity. The HN molecule, in addition to receptor binding, has important functions in receptor cleavage (mediated by its neuramidase enzyme), and in fusion promotion (acting in concert with the viral fusion protein (F)). The specific objectives of this proposal are: AIM 1: HPF3 RECEPTOR IDENTIFICATION USING STRATEGIES FOR PRODUCING SOLUBLE HPF3 HN PROTEIN. The objective of these experiments is to identify the cell surface molecules that serve as receptors for the HN protein. The approach to receptor identification proposed here will be undertaken in parallel with other approaches to receptor identification in progress in the laboratory. The main strategy for reaching this objective is the engineering of a recombinant HN protein that is biologically active, soluble, and secreted into the media. This recombinant protein will be used as a probe in binding studies for the identification of the HPF3 -HN receptor on mammalian cell surfaces. AIM 2: ANALYSIS OF FUNCTIONS OF HPF3 HN USING RECOMBINANT HN-GFP PROTEINS. To elucidate both the viral HN component of the virus-host interaction, we will evaluate the role of HN in viral entry and fusion using HPF3 HN variants that are altered in receptor binding or fusion promotion. The main strategy of this aim is to analyze the biological activities of HN using a reagent that allows in situ localization and quantification of the molecule. Our first hypothesis is that individual amino acid substitutions in the variant virus' HN molecules are responsible for the different neuraminidase and receptor binding capabilities of the variant HN molecules. Our second hypothesis in this aim is that continual expression of HN on the surface of infected cells results in receptor depletion on the surface of the mammalian cells, due to neuraminidase cleavage of the sialic acid molecules necessary for virus - cell interaction. The training objective of this application is to acquire both the formal didactic training and the in-depth laboratory experience that will allow me to develop into a first-rate pediatrician-investigator.